1. We focus here on the issue of how
stimulation patterns are to be divided into types.
Responses—primarily utterances—are also specific events
which need to be grouped into types if we are to correlate them with
stimulation patterns. We shall, however, ignore that issue here.

2. Somewhat less roughly: A is
perceptually similar to B than it is to C if A
evokes a response which has been conditioned to stimulation patterns
receptually similar to B but does not evoke a response
conditioned to patterns receptually similar to C.

3. Extensionality also requires that
replacing a singular term in a sentence by another with the same
reference leave the truth-value of the sentence unchanged. The
requirement perhaps has more evident plausibility than the others but
it plays no role in Quine’s fully regimented theory because in
that theory singular terms are eliminated by the technique of
Russell’s analysis of definite descriptions. Given weak and
plausible assumptions, the three requirements stand or fall together;
see, for example, Quine, 1995, 91.